1. Field of the Invention
The present invention relates to an optical head for writing and/or reading information on and/or from an optical record medium comprising a semiconductor laser for emitting a laser beam, a light converging means for converging said laser beam and projecting the thus converged laser beam onto the optical record medium, and a photodetecting means for receiving the laser beam reflected from the optical record medium by means of said light converging means.
2. Related Art Statement
The optical head of the type mentioned above has been widely used for writing and/or reading information on and/or from an optical record medium such as optical disk and optical card.
In Japanese Patent Application Laid-open Publication Kokai Sho 63-32743, there is disclosed a known optical head having a semiconductor laser. As shown in FIG. 1, the optical head includes a semiconductor laser 1 for emitting a diverging laser beam and a lens 2 for converging the laser beam. The laser beam is then transmitted through a hologram 3 and then is converted into a parallel laser beam by means of a collimator lens 4. The parallel laser beam is then made incident upon an objective lens 5 and is projected onto an optical record medium 6 such as a magneto-optical disk as a very fine spot.
The laser beam reflected by the magneto-optical disk 6 is made incident upon the hologram 3 by means of the objective lens 5 and collimator lens 4. Then there are produced plus and minus first order (.+-.1-order) diffracted laser beams having astigmatism contained therein. These .+-.1-order diffracted laser beams are then made incident upon photodetectors 9 and 10 by means of a polarizing lens 7. The photodetectors 9 and 10 are formed on a substrate 8 and each of them has four divided light receiving regions. An information signal may be obtained from a difference between output signals of the photodetectors 9 and 10 and at the same time a focusing error signal can be obtained by the well-known astigmatism method.
FIGS. 2 and 3 illustrate another known optical head including a semiconductor laser. A laser beam emitted by a semiconductor laser 1 is converted into a parallel laser beam by means of a collimator lens 4 and then is made incident upon an objective lens 5 via a polarizing beam splitter 11. It should be noted that the incident laser beam is of P-polarized beams, so that it is transmitted through the polarizing beam splitter.
The laser beam is then made incident upon a magneto-optical disk 6 by means of an objective lens 5. The laser beam reflected by the magneto-optical disk 6 is made incident upon the polarizing beam splitter 11 by means of the objective lens 5 and is reflected thereby. A polarizing direction of the laser beam reflected by the polarizing beam splitter 11 is rotated by 45 degrees by means of a half-wavelength plate 16 and is then made incident upon a trapezoidal prism 13 by means of a lens 12, said prism having a function for dividing polarized light. The prism 13 has a polarization beam splitting plane 13a which transmits P-polarized component but reflects S-polarized component. The P-polarized component transmitted through the polarization beam splitting plane 13a is made incident upon a photodetector 15a having three divided light receiving regions and the S-polarized component reflected by the polarization beam splitting plane 13a is made incident upon a photodetector 15b also having three light receiving regions. The photodetectors 15a and 15b are formed on a common substrate 14. An information signal can be reproduced by a difference between output signals from the photodetectors 15a and 15b and the focusing error signal can be obtained by the known beam size method.
In this known optical head, the P-polarized laser beam emitted from the semiconductor laser 1 is made incident upon the polarizing beam splitter 11, and thus information about the Kerr rotation contained in the laser beam reflected by the magneto-optical disk 6 is borne by the S-polarized laser beam. Therefore, in order to obtain the information signal having large amplitude and high C/N, it is advantageous to introduce in an efficient manner the S-polarized component into the photodetectors 15a and 15b. Therefore, it is desired to make the reflectance of the polarizing beam splitter 11 for the S-polarized light 100%. To this end, in this known optical pick-up head, transmittance T and reflectance R of the polarizing beam splitter 11 for the P- and S-polarized components are set to, for instance Tp 80%: Ts 0%: Rp 20%: Rs 100%.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-307759 published on Nov. 19, 1993, there is disclosed an optical pick-up head as shown in FIGS. 4 to 7. The optical pick-up head comprises a block like unit 16 of semiconductor laser, photodetectors and holograms and a laser beam emitted by the unit 16 is made incident upon an optical record medium 19 by means of a stop 17 and an objective lens 18. The laser beam reflected by the record medium 19 is then made incident upon the unit 16 by means of the objective lens 18 and stop 17.
As shown in FIG. 5, the unit 16 comprises a substrate 20 on which a semiconductor substrate 21 is provided, and a hologram element 22 is arranged on the substrate 20 via a spacer 23. On the semiconductor substrate 21 there is provided a semiconductor laser 24, and as best shown in FIG. 6 in a surface of the semiconductor substrate there are formed photodetectors 25, 26 and 27 and 28, 29 and 30 on both sides of the semiconductor laser 24 viewed in a tracking direction x, each of said two sets of photodetectors being aligned in a track direction y. The middle photodetectors 26 and 29 have three light receiving regions 26a, 26b and 26c and 29a, 29b and 29c which are divided along lines extending in the direction x. As illustrated in FIG. 7, in the front surface of the semiconductor substrate 21, there is formed a recess 21a with an inclined side wall 21b by etching, and the semiconductor laser 24 is placed on the bottom of the recess 21a. The inclined side wall 21b is polished as a mirror surface, so that a laser beam emitted from the semiconductor laser 24 in a direction parallel with the surface of the semiconductor substrate 21 is reflected by the inclined side wall 21b into a direction which is normal to the surface of the semiconductor substrate 21.
As depicted in FIG. 5, the hologram element 22 comprises on its one surface gratings 22a for dividing the laser beam emitted by the semiconductor laser 24 into three laser beams, i.e. 0-order beam (main beam), +1-order beam and -1-order beam (sub-beams). The hologram element 22 further comprises on its other surface a hologram pattern 22b for diffracting an incident beam and giving the +1-order and -1-order laser beams opposite focal powers.
The laser beam emitted by the semiconductor laser 24 is reflected by the inclined side wall 21b and is then divided by the gratings 22a into the single main beam and two sub-beams, these three beams being made incident upon the optical record medium 19 by means of the stop 17 and objective lens 18. The three laser beams reflected by the record medium 19 are made incident upon the hologram pattern 22b by means of the objective lens 18 and stop 17 and are diffracted thereby. .+-.1-order beams of the main beam are received by the middle photodetectors 26 and 29, respectively, .+-.1-order beams of one of the two sub-beams are received by the photodetectors 25 and 28, respectively and .+-.1-order beams of the other sub-beam are received by the photodetectors 27 and 30, respectively. The focusing error signal is derived from outputs of the photodetectors 26 and 29 in accordance with the beam size method, and the tracking error signal is obtained by output signals from the photodetectors 25, 27, 28 and 30 on the basis of the three beam method.
In the known optical head shown in FIG. 1, the laser beam reflected by the magneto-optical record disk 6 is made incident upon the photodetectors 9 and 10 after being diffracted by the hologram 3. The hologram 3 is of a thin type in which a depth of the recesses is smaller than a wavelength, and thus the diffraction efficiency for the .+-.1-order diffracted beams (an amount of .+-.1-order diffracted light/an amount of incident light) is not dependent upon the polarizing condition, so that a polarized component containing information about the Kerr rotation could not be completely introduced onto the photodetectors 9 and 10. Therefore, the magneto-optical information signal has a small amplitude, and thus C/N of the signal becomes decreased.
In order to mitigate the above mentioned drawback, it is considered that the diffraction coefficient for the .+-.1-order diffracted beams of the hologram 3 is increased, but then the diffraction coefficient for the 0-order beam (an amount of the 0-order light/an amount of incident light) would be deceased and a necessary laser power for writing information might become high and a problem of heat generation and so on might occur.
In the known optical head illustrated in FIGS. 2 and 3, the laser beam reflected by the magneto-optical record medium 6 is made incident upon the photodetectors 15a and 15b by means of the polarizing beam splitter 11, and therefore the polarized component bearing the information about the Kerr rotation could be effectively received by the photodetectors. However, the optical path is bent at right angles by the polarizing beam splitter 11, and thus a size of the optical head is liable to be large. Further, the semiconductor laser 1 and the photodetectors 15a, 15b are formed separately form each other, they are liable to be deviated from each other due to temperature variation and secular variation, and thus a reliability of the optical head is low.
In the optical pick-up head shown in FIGS. 4 to 7, the semiconductor laser 24, photodetectors 25-30 and hologram element 22 are all integrated into the single unit 16, and thus the optical head can be made small in size and the reliability can be improved. However, there is not disclosed a concrete construction for deriving the magneto-optical information signal.
Therefore, a primary object of the present invention is to provide a novel optical head for mitigating the above mentioned drawbacks of the known optical head, in which the head can be made small in size, a high reliability can be attained, the decrease in C/N can be suppressed and the magneto-optical information can be read out stably.
According to one aspect of the present invention, an optical head comprises:
a semiconductor substrate having a surface;
a semiconductor laser arranged on said surface of the semiconductor substrate for emitting a laser beam;
a light receiving means including a plurality of photodetectors formed in said surface of the semiconductor substrate;
a collimator lens for converting said laser beam emitted by said semiconductor laser into a substantially parallel laser beam;
an objective lens for converging said substantially parallel laser beam onto a magneto-optical record medium;
a beam dividing means arranged between said collimator lens and said objective lens for dividing a laser beam reflected by said magneto-optical record medium into first and second return beams; and
a polarization beam splitting means arranged between said semiconductor laser and said collimator lens such that an integral unit is formed together with said semiconductor substrate, and including gratings for diving the laser beam emitted by said semiconductor laser into a main beam which is used to write or read information on or from said magneto-optical record medium and sub-beams which are used to derive a tracking error signal, a first hologram pattern for diffracting said first return beam, a second hologram pattern for diffracting said second return beam, and a polarization beam splitting plane arranged substantially in parallel with an optical axis of a zero order beam emanating from said second hologram pattern for splitting +1-order and/or -1-order diffracted beam emanating from said second hologram pattern in accordance with a polarizing direction of the beam; whereby +1-order and -1-order diffracted beams emanating from said first hologram pattern and beams emanating from said polarization beam splitting plane are separately received by said plurality of photodetectors of said light receiving means.
According to another aspect of the invention, an optical head comprises:
a semiconductor substrate having a surface;
a semiconductor laser arranged on said surface of the semiconductor substrate for emitting a laser beam;
a light receiving means including a plurality of photodetectors formed in said surface of the semiconductor substrate;
a collimator lens for converting said laser beam emitted by said semiconductor laser into a substantially parallel laser beam;
an objective lens for converging said substantially parallel laser beam emanating from said collimator lens onto a magneto-optical record medium;
a beam dividing means arranged between said collimator lens and said objective lens for dividing a laser beam reflected by said magneto-optical record medium into first and second return beams; and
a polarization beam splitting means arranged between said semiconductor laser and said collimator lens such that an integral unit is formed together with said semiconductor substrate, and including a first hologram pattern for diffracting said first return beam, a second hologram pattern for diffracting said second return beam, a third hologram pattern for diffracting a zero order beam emanating from said first hologram pattern, and a polarization beam splitting plane arranged substantially in parallel with an optical axis of a zero order beam emanating from said second hologram pattern for splitting +1-order and/or -1-order diffracted beams emanating from said second hologram pattern in accordance with a polarizing direction of the beam; whereby 1-order and -1-order diffracted beams emanating from said first hologram pattern, beams emanating from said polarization beam splitting plane and +1-order and/or -1-order diffracted beam emanating from said third hologram pattern are separately received by said plurality of photodetectors of said light receiving means.
According to another aspect of the invention, an optical head comprises:
a semiconductor substrate having a surface;
a semiconductor laser arranged on said surface of the semiconductor substrate for emitting a laser beam;
a light receiving means including a plurality of photodetectors formed in said surface of the semiconductor substrate;
an objective lens converging the laser beam emitted by said semiconductor laser onto a magneto-optical record medium; and
an optical block arranged to form an integral unit together with said semiconductor substrate and including a hologram for diffracting a return beam reflected by said magneto-optical record medium and a polarization beam splitting plane arranged in substantially parallel with an optical axis of a zero order beam emanating from said hologram for splitting a pupil of said hologram; wherein return beams diffracted by said hologram and split by said polarization beam splitting plane are separately received by said plurality of photodetectors of said light receiving means.
In Japanese Patent Application Laid-open Publication Kokai Hei 4-248134 published on Sep. 3, 1992, there is disclosed another known optical head. As shown in FIGS. 8 and 9, this known optical head comprises a semiconductor substrate 31 in which first and second photodetectors 32 and 33 are formed. On the semiconductor substrate 31 there are arranged semiconductor laser 35 and reflection mirror 36. A laser beam emitted by the semiconductor laser 35 is reflected by the mirror 36 upwardly toward an optical record medium 40 by means of hologram element 38 and objective lens 39.
The hologram element 38 has formed thereon a hologram pattern 38a on a surface facing the objective lens 39 and gratings 38c on a surface facing the substrate 31. The gratings 38c serve to divide the laser beam emitted by the semiconductor laser 35 into a main beam and two sub-beams, these three beams being made incident upon the optical record medium 40 by means of the objective lens 39. The three laser beam reflected by the optical record medium 40 are made incident upon the hologram element 38 by means of the objective lens 39. Each of these beams is divided into .+-.1-order beams having opposite refraction powers. 1-order beams are received by the first photodetector 32 and -1-order beams are received by the second photodetector 33. The photodetector 32 comprises three light receiving elements 32a, 32b and 32c for receiving the three +1-order beams, and the middle light receiving element 32b for receiving the +1-oder beam of the main beam has three light receiving regions 32d, 32e and 32f which are divided in the same direction as the diffracting direction of the hologram pattern 38a. Similarly, the photodetector 33 comprises three light receiving elements 33a, 33b and 33c for receiving the three -1-order beams, and the middle light receiving element 33b for receiving the -1-oder beam of the main beam has three light receiving regions 33d, 33e and 33f which are divided in the diffracting direction of the hologram pattern 38a.
Then, the focusing error signal can be derived from output signals of the light receiving regions 32d, 32e and 32f of the light receiving element 32b for receiving the +1-order beams of the main beam and output signals of the light receiving regions 33d, 33e and 33f of the light receiving element 33b for receiving the -1-order beams of the main beam on the basis of the beams size method. Further the tracking error signal is derived from output signals from the light receiving elements 32a and 32c for receiving the +1-order beams of the two sub-beams and output signals from the light receiving elements 33a and 33c for receiving the -1-order beams of the two sub-beams in accordance with the three beam method. In FIGS. 8 and 9, a direction in which information tracks extend is denoted by x and a tracking direction perpendicular to the direction x is represented by y.
This known optical head can be constructed from a smaller number of parts, and further the semiconductor laser 35 and photodetectors 32 and 33 are provided on the same semiconductor substrate 31, so that the optical head can be made small in size. Moreover, even if the diffraction angles by the hologram pattern 38a for the .+-.1-order diffracted beams are changed due to the variation in the wavelength of the laser beam emitted from the semiconductor laser 35, the images on the photodetectors 32 and 33 move in the direction which is parallel with the dividing lines of the photodetectors, and therefore the focusing error signal is hardly affected by the fluctuation of the wavelength. Furthermore, even if the spots on the photodetectors 32 and 33 are shifted due to errors in machining various parts and in assembling, the +1-order beams and -1-order beams are shifted in the same manner, and thus it is possible to cancel out off-sets of the focusing error signal from the +1-order beams and of the focusing error signal from the -1-order beams. In this manner, the signal detection can be performed stably and adjustment of various parts during the assembling can be simplified.
This known optical head could be applied to the compact disk device and once-write type disk device, but could not be utilized for the magneto-optical disk device, because the optical head does not comprise the polarizing splitting means.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-120755 published on May 18, 1993, there is disclosed another optical head which could be used for the magneto-optical disk. As depicted in FIGS. 10 and 11, this optical head comprises a semiconductor substrate 31 in which first, second and third photodetectors 32, 33 and 34 are formed. On the semiconductor substrate 31 there are arranged semiconductor laser 35, reflection mirror 36 and polarizing beam splitter 37. A laser beam emitted by the semiconductor laser 35 is reflected by the mirror 36 upwardly toward an optical record medium 40 by means of hologram element 38 and objective lens 39. It should be noted that the optical record medium 40 is formed by the magneto-optical disk. On one surface of the hologram element 38 there are formed hologram patterns 38a and 38b and on the other surface there are formed gratings 38c. The function of the hologram element 38 is substantially same as that of the known optical head illustrated in FIGS. 8 and 9. Further the construction of the first and second photodetectors 32 and 33 are similar to that of the optical head shown in FIGS. 8 and 9 and receive the three 1-order beams and three -1-order beams, respectively.
The third photodetector 34 comprises two light receiving regions 34a and 34b which are inclined by 45 degrees with respect to the directions x and y. On the third photodetector 34, there is arranged the polarizing beam splitter 37 such that a polarization beam splitting plane 37a of multiple coatings of dielectric films is inclined by 45 degrees with respect to the direction y.
The focusing error signal can be obtained from output signals of the light receiving regions 32d, 32e and 32f of the light receiving element 32b for receiving the +1-order beams of the main beam and output signals of the light receiving regions 33d, 33e and 33f of the light receiving element 33b for receiving the -1-order beams of the main beam on the basis of the beams size method, and the tracking error signal can be derived from output signals from the light receiving elements 32a and 32c for receiving the +1-order beams of the two sub-beams and output signals from the light receiving elements 33a and 33c for receiving the -1-order beams of the two sub-beams in accordance with the three beam method. P-polarized component of the +1-order beam of the main beam which is transmitted through the plane 37a of the polarizing beam splitter 37 is received by the light receiving region 34a and S-polarized component reflected by the plane 37a is received by the light receiving region 34b. Now it is assumed that output signals from these light receiving regions 34a and 34b are denoted by Ia and Ib, respectively. Then, the magneto-optical information signal S is obtained as a difference between these signals, i.e. S=Ia-Ib.
In this optical head, since the polarizing beam splitter 37 is provided, it is possible to apply to the magneto-optical disk. However, an operation of mounting the small polarizing beam splitter having a complicated shape on the photodetector 34 is very cumbersome and requires a human skill and a long time, which apparently increases a cost of the optical head.
It is another object of the invention to provide a novel and useful optical head which can be effectively applied to the magneto-optical disk and can be easily assembled without difficult adjustment in a less expensive manner.
According to further aspect of the invention, an optical head comprises:
a semiconductor laser for emitting a laser beam;
a converging means for converging said laser beam emitted by the semiconductor laser onto a magneto-optical record medium;
a light receiving means for receiving a return beam reflected by said magneto-optical record medium and converged by said converging means;
a diffracting means arranged between said semiconductor laser and said converging means for diffracting said return beam reflected by said magneto-optical record medium; and
a polarization beam splitting means having a polarization beam splitting plane which is substantially in parallel with an optical axis of a zero order beam of the return beam emanating from said diffracting means and splits diffracted beams emanating from said diffracting means in accordance with polarizing directions of said diffracted beams.
In this optical head according to the invention, the polarization beam splitting plane is arranged to be substantially in parallel with the optical axis of the zero order beam emanating from the diffracting element, and thus an incident angle of an incident beam upon the polarization beam splitting plane can be made large. Therefore, the polarization beam splitting plane can be manufactured easily and the positional adjustment of the light receiving elements with respect to the polarization beam splitting element can be simplified. Moreover, the diffracting element and the polarization beam splitting element may be formed into a single unit, so that the optical head can be made compact in construction and cheap in cost.
The present invention also relates to a light emitting and receiving device for use in the optical head.
FIG. 12 is a perspective view showing a known light emitting and receiving device, which is described in Japanese article "Extended Abstracts (40th Spring Meeting, 1993) 29p-c-12; The Japan Society of Applied Physics and Related Societies No. 3)" held on March 39 to Apr. 1, 1993 at Tokyo, Japan. In this known device, in a surface of a silicon semiconductor substrate 41, there are formed photodetectors 42 and 43 and a recess 44 having a side wall 45 which is inclined by 45 degrees with respect to the surface of the substrate, and on a bottom of the recess is arranged a semiconductor laser 46. It should be noted that inclined side wall 45 serves as a micromirror for reflecting a laser beam emitted from the semiconductor laser 46 in a direction parallel with the surface of the silicon substrate 41 and the reflected laser beam propagates in a direction perpendicular to the surface of the substrate. In this manner, the light emitting and receiving device is realized which comprises a surface emitting type laser element and a photoelectric element.
In Japanese Patent Application Laid-open Publication Kokai Hei 3-187285, there is disclosed a multibeam semiconductor laser device in which a plurality of laser diodes are integrated as a monolithic device. In Japanese Patent Application Laid-open Publication Kokai Hei 3-112184, there is shown a semiconductor laser device in which three semiconductor lasers are integrated on a sub-mount such that light emitting points are deviated from each other.
In the known light emitting and receiving device illustrated in FIG. 12, only one semiconductor laser is provided, so that this device could not be utilized in such a case that a plurality of semiconductor lasers have to be arranged close to each other. In another known laser device disclosed in the Japanese Patent Application Laid-open Publication Kokai Hei 3-187285, there is a limitation that the light emitting points have to be aligned along a line perpendicular to an optical axis. Further, in another known semiconductor laser device described in the Japanese Patent Application Laid-open Publication Kokai Hei 3-112184, it is possible to arrange the light emitting points in a three-dimensional manner, but two semiconductor lasers have to be positioned with respect to the middle semiconductor laser, so that if these semiconductor lasers have to be arranged such that they are separated from each other by comparatively large distances, a precision of the positioning might be decreased. Moreover, the three semiconductor lasers on the sub-mount are positioned with respect to optical axes by utilizing a difference in a thickness of base substrates, and thus the precession of the positioning in a direction perpendicular to the optical axis is affected by the precision of manufacturing the base substrates.
Therefore, the present invention has for its object to provide a novel and useful light emitting and receiving device, in which light emitting points of a plurality of semiconductor lasers can be arranged in a three-dimensional manner, while the precision of positioning of the semiconductor lasers and the precision of positioning the semiconductor lasers and light receiving element can be improved.
According to the invention, a light emitting and receiving device comprises:
a semiconductor substrate having a surface;
a plurality of recesses formed in said surface of the semiconductor substrate;
at least one photodetector formed in said surface of the semiconductor substrate at a position outside said recesses; and
a plurality of semiconductor lasers arranged on bottoms of said recesses for emitting laser beams.
According to one aspect of the invention, a depth of at least one recess differs from that of the remaining recesses or a distance between at least one inclined side wall of a recess and an end surface of a semiconductor laser arranged in the relevant recess differs from a distance between at least one inclined side wall of other recess and an end surface of a semiconductor laser arranged in the relevant recess. Then, it is possible to obtain a surface emitting type laser and in which a plurality of light emitting points can be precisely arranged in a three-dimensional manner and can be precisely positioned with respect to the light receiving element.
According to another aspect of the invention, a light emitting and receiving device comprises:
a semiconductor substrate having at least one recess formed in one surface thereof;
at least one photodetectors formed in said surface at a position outside said recess; and
a plurality of semiconductor lasers arranged on bottoms of said recess for emitting laser beams.
In this light emitting and receiving device according to the invention, a semiconductor laser is arranged such that its end surface is not in the same plane as that of other semiconductor laser, so that a surface emitting type laser can be realized in which a plurality of light emitting points can be precisely arranged in a three-dimensional manner and can be precisely positioned with respect to the light receiving element.